Invasive species

In a healthy ecosystem the species keep each other in check. Every animal has enemies, every plant has competitors. If humans now bring in a species from another part of the world, on purpose or by accident, the balance can tip over. Such newly arrived species are called invasive species when they spread strongly and cause damage.

Why they spread so strongly

In the new place the introduced species often lacks its old enemies and diseases. Nobody eats it, nothing slows it down. If it then also finds enough food, its numbers grow very fast. The native species come under pressure: they lose food and habitat to the new competitor and become fewer and fewer.

Examples and damage

In 1859 a settler in Australia released about two dozen rabbits for hunting. Without natural enemies they multiplied into the millions within a few decades and grazed whole landscapes bare. The raccoon from North America now lives in many European forests, where it raids the nests of native birds. Invasive species can throw whole food chains into disorder and push out species that exist only in one place.

What can be done about it

The best thing is to prevent the introduction from the start. Goods and ships are checked so that no stowaways travel along, and the ballast water of large ships is treated, because mussels, crabs, and algae ride halfway around the world in it. If a species has already settled, people try to keep its numbers small. Getting rid of it entirely is usually no longer possible, so caution is better than late regret.